1. Field of the Invention
The present invention relates to a light receiving device for an optical communication system, more specifically, to a light receiving device suitable for use in a wavelength-division multiplexed optical communication system.
2. Description of the Related Art
FIG. 8 is a block diagram of a light receiving device for use in the conventional optical communication system.
In this light receiving device, a light signal transmitted through an optical fiber enters an optical bandpass filter 100, and is filtered by the filtering characteristics of the optical bandpass filter 100. Further, the light signal passed through the optical bandpass filter 100 enters an optical bandpass filter 101, and is filtered by the filtering characteristics of the optical bandpass filter 101.
Thus, the light signal is filtered by the optical bandpass filters 100, 101 into a signal with only a specific bandwidth, and the signal enters a light receiver 102 to be demodulated into a modulation signal.
Furthermore, the reason that the optical bandpass filters 100 and 101 are cascaded is as follows.
To realize a high-capacity communication by the optical communication system, there is a wavelength-division multiplexed optical communication system proposed. In such a system, the wavelength dependence of gain of an amplifier used in repeaters produces the difference in signal intensity between wavelengths. To suppress this influence and to reduce the difference of waveform distortion in wavelengths by chromatic dispersion in an optical fiber transmission line, it is necessary to narrow spacing between wavelengths (channel spacing) as much as possible.
However, it is generally said that a feasible full width at half maximum of the optical bandpass filter is about 1 nm (about 0.5 nm even for a special experiment purpose). Therefore, to narrow spacing between wavelengths will lead to impossibility of separating a desired wavelength (channel) signal from the wavelength-division multiplexed signal. Thus, cascading optical bandpass filters and thereby narrowing the overall passband of wavelengths makes it is possible to separate the desired wavelength (channel) signal from the wavelength-division multiplexed signal with wavelength spacing narrowed. There are the following problems on a conventional light receiving device in which the optical bandpass filters are cascaded.
1. To narrow the effective passband of a multi-stage cascaded optical bandpass filters the center wavelengths of the optical bandpass filters must all be strictly at the same wavelength. This is a very difficult adjustment to make.
2. If there is a minute change in the properties of the optical bandpass filters due to a change in external environments such as temperature, the center wavelengths of the filters drift so that the filters cannot satisfy the desired bandpass characteristics.
3. Furthermore, to interpose an optical bandpass filter attenuates received signal and reduces the possible transmission distance for the optical communication system. Cascading optical bandpass filters further increases the attenuation of the received signals, with the result that the possible transmission distance of the optical communication system is increasingly reduced.
4. Furthermore, when a frequency modulated or phase modulated light enters an optical bandpass filter, the intensity noise components contained in the foregoing modulated light cannot be removed at the optical fiber output due to absence of the optical limiter function.
5. Moreover, if the optical bandpass filters are designed to be cascaded, the shortest wavelength spacing of the multiplexed signals is about 1 mm, and to arrange the wavelengths closer is almost impossible.
By way of an example, FIG. 9 shows a spectrum when a light of four wavelengths multiplexed with a wavelength spacing of 0.5 nm passes through a filter in which optical bandpass filters having full width at half maximums of 0.5 nm and 3 nm are cascaded (Technical Report of IEICE optical communication system, OCS94-47 P.33.about.38 "Tranmission experiment on high-density wavelength-division multiplexed systems combined with polarization multiplexing" INSTITUTE OF ELECTRONICS, INFORMATION and COMMUNICATION ENGINEERS, 1994-10). From this FIG. 9, compared to the light intensity of the wavelength (channel) to be extracted located at the second from the right end, the light intensity of the adjacent channel can be observed to be attenuated by only a only 6 dB. If is clearly seen that extracting the desired one without being interfered by the adjacent chancel is totally difficult.
A method to overcome this kind of problem is proposed in the aforementioned Technical Report of IEICE. According to this method, additional constructions such as one for forming an orthogonal polarization and one for separating polarized waves are required. Furthermore, even if this method is adopted, the improvement of crosstalk was only about 6.5 dB, and the attenuation of light intensity of the interfering channel was only about 12.5 dB.